[CVE-2026-98840][MODERATE 7.0] vc_screen: reload vc pointer before if (ret) in vcs_write() to avoid UAF [ Upstream

[CVE-2026-98840][MODERATE 7.0] vc_screen: reload vc pointer before if (ret) in vcs_write() to avoid UAF [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98840
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: vc_screen: reload vc pointer before if (ret) in vcs_write() to avoid UAF [ Upstream
Commit: 1485a3dd97ab1af2605243cabe6b604546d46dd1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1485a3dd97ab1af2605243cabe6b604546d46dd1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98840
Analysis date: Mon, 19 Oct 2026 04:58:56 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A race in `vcs_write()` can pass a freed `vc_data` object to `vcs_notifier()`, causing a confirmed two-byte slab use-after-free read and a possible local kernel crash, while no controlled reclaim, write primitive, disclosure, or privilege escalation is established.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-98840 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98840

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98840	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98840 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-362;CWE-367;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in vcs_write can leave a stale non-NULL vc_data pointer when copy_from_user fails after the virtual console was freed during the unlocked window. The stale pointer is later passed through vcs_scr_updated to vcs_notifier, which reads vc_num from freed slab memory and produces a confirmed use-after-free read. For the CVSS the PR:L value is used because triggering requires a local user or process with access to the relevant vcs and virtual terminal interfaces, while the exact device permissions depend on the system configuration. AC:H is used because successful triggering depends on winning a race between vcs_write and virtual console deallocation. The issue is not network reachable. Impact is at least a local denial of service through a kernel warning, Oops, or crash. Limited confidentiality or integrity impact is retained only in the paranoid score because slab reuse may affect the stale read, but no controlled write or demonstrated privilege escalation primitive is present. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 5.8 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 1.9 and ActionableScore 0 with primitive class CLASS_B and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	READ NULLPTR PRINTF-TTY UAF  KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 3
* Paranoid score: 4
* Applied conservative floor: 3 -> 4 under REAL UAF FLOOR 4.
* Applied paranoid floor: 3 -> 4 under REAL UAF FLOOR 4.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Real UAF. `vc_port_destruct()` can free `struct vc_data` while `vcs_write()` temporarily releases `console_lock`, after which the stale non-NULL pointer reaches `vcs_notifier()`.
* +1 Reliable kernel fault or strong DoS. KASAN confirms a slab use-after-free read, and an uninstrumented kernel may produce an Oops or crash.
* +0 Structured stale-object leverage. The stale object is structured, but the demonstrated operation only reads the two-byte `vc_num` field for a notifier comparison. No pointer traversal, callback dispatch, write, or lifetime mutation through reclaimed contents is established.
* +0 Repeated release or refcount mutation. The stale path does not free, put, or decrement a reference on `vc`; it performs a read only.
* +0 Local unprivileged trigger. Local interfaces are identified, but the supplied evidence does not establish that one below-host-admin actor can both access the relevant VCS device and cause `VT_DISALLOCATE`.
* +0 Race penalty. A concurrent free during `copy_from_user()` is required, but the evidence does not establish that the timing is unusually narrow or difficult to repeat.
* +0 Additional availability. The same UAF fault is not counted twice.
* +0 Confidentiality or integrity. The stale value is not shown to be disclosed or used for a security-sensitive modification.
* +0 B5-R. There is no heap OOB write, retry-accumulated offset, or later consumption of OOB-corrupted state.
* +0 B3-PENDING. No shared-backing or file-backed fragment write exists.
* +0 V33 or V34 candidate. No attacker-influenced write into active structured state is established.
* +0 B4-PENDING. No authorization guard changes from DENY to ALLOW.

Controlled reclaim, arbitrary read or write, attacker-shaped replacement contents, repeated lifetime mutation, authorization bypass, and remote reachability are not demonstrated.

3. Reachability analysis

The trigger requires a local `vcs_write()` operation that has already written some data, an unsuccessful or partial `copy_from_user()`, and concurrent virtual-console destruction through `VT_DISALLOCATE` and `vc_port_destruct()`.

The supplied evidence does not resolve whether a single ordinary user can perform the complete sequence under typical device permissions. Access to `/dev/vcs*`, ownership of the controlling terminal, and permission for the relevant VT ioctl can vary by system. Therefore `HostAdminRequired=UNKNOWN` rather than assuming either host administration or ordinary unprivileged reachability.

Linux virtual consoles are host-global rather than an attacker-created user or network namespace facility, so the complete operation is not namespace-scoped. No network trigger, special hardware, or guest boundary is involved. The race may be attempted repeatedly by cooperating local tasks, but its practical reliability is not established strongly enough for either a reachability bonus or a hard-race penalty.

4. Primitive and severity interpretation

The exact primitive is a confirmed two-byte slab use-after-free read from `vc_data->vc_num`. `vc_data` is an active structured object, but this particular stale-use path is shallow. The freed pointer is passed to `vcs_scr_updated()` and then `vcs_notifier()`, where one scalar field is read and compared.

This is not a concrete repeated-release case. The first operation frees the object, but the later stale operation does not mutate its refcount or lifetime state. No second `put`, `free`, or equivalent operation exists, so the repeated-release signal and FLOOR 5 do not apply.

The demonstrated consequence is a KASAN report and plausible kernel Oops or crash. A stale `vc_num` obtained from reused slab storage could also cause an incorrect notifier match, but no protected-state modification or independently useful exploitation primitive is shown. Generic allocator reuse is insufficient to score controlled reclaim or privilege escalation.

`PrimitiveClass=CLASS_B` is appropriate because an actual post-lifetime dereference is established. `ImportantCandidate=NO` because the stale use is a constrained read without meaningful unresolved write, callback, ownership, refcount, disclosure, or boundary-bypass leverage. `AutoDowngradeAllowed=YES` because the evidence supports a constrained local UAF and DoS rather than an Important-preserving corruption primitive.

5. One-sentence report phrase

A race in `vcs_write()` can pass a freed `vc_data` object to `vcs_notifier()`, causing a confirmed two-byte slab use-after-free read and a possible local kernel crash, while no controlled reclaim, write primitive, disclosure, or privilege escalation is established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The UAF is real and should be reviewed, but the demonstrated stale operation is a shallow scalar read with no supported Important-grade exploitation leverage.

7. Call-site confidence

Call-site confidence: high

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: vc_screen: reload vc pointer before if (ret) in vcs_write() to avoid UAF [ Upstream
Commit: 1485a3dd97ab1af2605243cabe6b604546d46dd1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1485a3dd97ab1af2605243cabe6b604546d46dd1

Changed files:
  drivers/tty/vt/vc_screen.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1485a3dd97ab1af2605243cabe6b604546d46dd1

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-98840 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98840

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: unknown
  Paranoid ActionableScore: 4

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).